Fix infringement on user name space reported by Bruno Haible.

* data/yacc.c (YYSIZE_T): Define first, so that later decls can use it.
Prefer GCC's __SIZE_TYPE__ if available, so that we don't infringe on
the user's name space.
(alloca): Include <stdlib.h> to get it, if it's not built in.
(YYMALLOC, YYFREE): Define only if needed.
(malloc, free): Declare, but only if needed, as this infringes on
the user name space.